There is a steadily increasing need for economical and reliable apparatus to measure the turbidity of fluid samples. Turbidity, which denotes the tendency of a sample to scatter and absorb light rather than transmit it, has become a commonly accepted criterion of water quality. Turbidity is typically caused by the presence of suspended matter such as clay, mud, algae, rust, bacteria, and calcium carbonate. Many processes for the treatment of water for potable or industrial usage depend upon turbidimeters for efficient and economical operation. For example, turbidimeters are used to automatically control chemical dosage rates and to monitor the effluent from a filter. With increasing emphasis on pollution control, the need for economical and reliable turbidimeters has increased substantially in recent years.
While a variety of complex optical systems have been used to measure turbidity in a carefully controlled laboratory environment, many of these optical systems are too delicate for reliable field operation. Turbidimeters for field use, particularly for prolonged use in installations for automatic control, are typically subject to shock in transport, changes in the color of the fluid sampled, the buildup of turbid films on the sample cell, and variations in the intensity of the light source.